TE probe installation

On Friday, December 5, 2014 11:26:06 AM UTC-5, Jim Lewis wrote:
Thank you JP. I'll dig into the info today.

Forgive me for this simple-minded question. What pressure (static, dynamic, total) is provided to the variometer by the TE probe? I have read that a TE probe must create a pressure inverse but equal in maginitude to the dynamic pressure at the pitot. This confuses me. I thought the pitot 'receives' total pressure not just dynamic pressure.

I do understand that a variometer is a flow meter that responds to the flow of air between a capacity and the pressure sensed by the static ports. Does the TE probe provide this static pressure (offset by changes in dynamic pressure created by the glider varying airspeeds) or is the TE probe providing dynamic pressure to its side of the variometer?

I am trying to understand the TE process without being able to actually measure the pressures involved.

Thank you.

The pitot does indeed have an absolute pressure equal to local static pressure plus dynamic pressure. An airspeed indicator compares this to static, so far all intents and purposes, static pressure is meaningless.
A FLOW type variometer uses TE pressure(Pitot x -1) to compensate the variometer for velocity change. Static is not a factor.
In transducer type varios, there is no flow. These devices use pitot compared to static, and TE compared to static to get a similar result. Some others do not use TE pressure and derive required input by inverting pitot signal electronically.
For most folks it is good enough top take this stuff on "faith". It does work.
UH

Thank you again UH. I do take this on faith since I understand the purpose of the total energy compensation and I can see its effects. Unfortunately, students sometimes ask 'how'. I feel I must avoid talking nonsense when they ask. So far I have just replied that I do not know 'how' but I do know 'what'. After several such experiences my own curiosity has led me to try to understand 'how'. I guess I can't help myself!

On Friday, December 5, 2014 5:20:55 PM UTC, Jim Lewis wrote:
Thank you again UH. I do take this on faith since I understand the purpose of the total energy compensation and I can see its effects. Unfortunately, students sometimes ask 'how'. I feel I must avoid talking nonsense when they ask. So far I have just replied that I do not know 'how' but I do know 'what'. After several such experiences my own curiosity has led me to try to understand 'how'. I guess I can't help myself!

For another excellent reference about pneumatic tubing and TE compensation see Part 1 of this paper by Dave Ellis (now of ClearNav and previously the designer of the Cambridge 302)

http://www.clearnav.net/Technical/Va...pneumatics.pdf

John Galloway

Thank you John. It's an excellent article.

On Friday, December 5, 2014 9:00:22 AM UTC-8, wrote:
A FLOW type variometer uses TE pressure(Pitot x -1) to compensate the variometer for velocity change. Static is not a factor.
In transducer type varios, there is no flow. These devices use pitot compared to static, and TE compared to static to get a similar result. Some others do not use TE pressure and derive required input by inverting pitot signal electronically.

Having built a couple of transducer variometers, when using TE probe compensation, I simply differentiate the absolute TE pressure, static is not used in that case...

Marc

Another simpler explanation can be found on this link:

http://bas.uk.net/variofaq.html

Dickie

On 04/12/2014 19:02, Jim Lewis wrote:
I have been trying to understand just how a TE probe functions.

Is a TE probe plumbed as the only static source for a variometer or is it plumbed together with the static ports that are usually placed on the sides of a glider?

Thank you.

On Friday, December 5, 2014 1:19:59 PM UTC-8, Dickie Feakes wrote:
Another simpler explanation can be found on this link:

http://bas.uk.net/variofaq.html

Dickie

Thank you Dickie. That's a very helpful explanation.

Most important thing in understanding how TE probe works is to understand that variometer does not care about the absolute pressure. It measures only pressure change over time. If you dive your glider, the static pressure rises. This is compensated by increasing negative pitot pressure (negative, because holes are at the wake of TE tube). These pressure components should be more or less equal and cancel each out, leaving only pressure change from aircraft polar sink and airmass movement.

Good point! I'm sad to say that it has taken me several days to remember that the variometer, unlike the altimeter, is not concerned with absolute pressure, only with the relative pressure on both sides of the variometer.

Thank you to all who have helped me sort out my understandings of how TE probes work.

My understanding has been blocked by my confusion over how a TE probe senses environmental static pressure. I know, dumb. This confusion led me down a dead end path of wondering just what pressure (total, dynamic, static) a TE probe presents to the static side of a variometer.

I read several souces on the subject and was further stumped by repeated statements that a TE probe must produce a pressure equal in magnitude but opposide in sign to the dynamic pressure sensed by the pitot probe. This just fed my confusion.

The veil has been lifted from my eyes - I hope. I understand now that the TE probe (and the pitot probe, for that matter) cannot help but sense static pressure - in addition to dynamic pressure. Static pressure, in a sense, is everywhere. It is, in a sense, the background pressure. The TE probe does sense static pressure; it cannot help but do so. But it also senses dynamic pressure (the inverse of it, actually). The TE probe 'adds' the inverse of dynamic pressure to the static pressure to moderate the static pressure changes that may accompany dynamic pressure changes. If the dynamic pressure is not changing the TE probe presents to the variometer the pressure it does sense - the unmodified static pressure, and so functions as a simple static probe. Again, in a sense, the TE probe, in the absence of dynamic pressure changes, 'adds' zero to the static pressure.

I hope I finally have this sorted out. If not, just shoot me.